The invention relates to a method and system in control of a coating Color recipe.
The chemical composition of coating mix in manufacture, that is, for example its pigment, binder and thickener composition and the amount of other additives, are determined on the basis of a coating Color recipe. The coating Color recipe determines the amounts and/or ratios of the various components of the coating mix. According to the Color coating recipe, the coating mix may contain, for example, 50 parts calcium carbonate, 50 parts kaolin, 10 parts latex and 0.1 parts lye, that is, the sum of the ratio of the components may be greater than 100. Control of the coating Color recipe in this connection refers to measures, whose purpose is to produce a new coating mix complying with the coating Color recipe, or to make a partly recycled coating mix as close as possible to the coating mix according to the coating Color recipe.
In the manufacture of coating mix, recycled mix, that is, for example, coating mix collected from the machine cycle, or recovered mix, is used as one component to reduce manufacturing costs. Concentrate, that is, mix recovered by filtering (for example by means of an ultrafilter or nanofilter) from collected waters containing coating, is also often used in coating mix manufacture. In addition, in coating change, that is, in connection with changing the coating Color recipe, the aim is to recover the old coating. Recovery takes place partly by pumping the coating mix from the machine cycle to a recovery tank. Coating mix is also conveyed to the recovery tank partly along with the washing waters. The dry matter content of the fraction obtained through recovery by these two methods may vary considerably, being typically between 0.5–65%. The collection methods and different methods for treating the fractions also cause chemical changes in the properties of the coating. For example, in ultrafiltration, small-molecule components, such as, for example, the dextrines of the starch contained in the coating mix, pass through the filter with the water, whereby the structure of the coating mix changes as the amount of small-molecule components changes.
The properties of the mix in the machine cycle are known to change in connection with the coating process. The changes are caused, among other things, by the migration of binder, water, optical brightener and other components to the base paper. Some of the changes are also due to the evaporation of water when the coating is in contact with air, for example, in the return trough and on the surface of the machine chest. The water balance of the machine cycle may also change due to the effect of external factors, such as, for example, cleaning showers and condensate waters.
Due to the effect among other things, of the changes and uncertainty factors described above, in known solutions the concentrate is typically almost always treated in terms of the calculation as a pigment. However, in addition to pigment, the concentrate also contains many other components, such as latex. If the chemical composition of the concentrate, that is, the amount of components contained in it, or their ratio, were known, then for example in coating mix manufacture, the amount of latex could be reduced, which would result in considerable cost savings.
Furthermore, changes in the properties of coating mix affect the quality of the final coating and the runnability of the coating process. Thus, for example, a decrease in the amount of thickener and latex will increase the viscosity of the coating mix, which will have a significant effect on the runnability of the coating process. Furthermore, it is also known that using concentrate in the manufacture of coating mix will decrease the viscosity of the finished coating mix.
Thus a need has arisen to measure the properties of coating mix in the machine cycle and during coating mix recovery. There has also arisen a need to control the properties of the coating mix to be made on the basis of the properties of the coating mix when ready and when used for coating.
European patent application EP 0 962 769 discloses a method and an apparatus for measuring the properties of a composition or component used in paper or board manufacture. According to the application, a sample taken of the composition is measured for dry matter content, viscosity and pH. A problem with the solution presented in the application is that it does not reveal the chemical composition of the composition. Thus, for example, the real cause of a change in viscosity of the composition cannot be discovered, which means that it is difficult to so remedy the problem effectively and efficiently.
Furthermore, for example, from the patent application WO 99/14579 there is known a method and device for measuring the properties of paper by directing an infrared beam onto the surface of a moving paper web. A beam that has passed through or been reflected from the web is divided into wavelength components by means of a spectrograph and the signals are measured for the various wavelength components, on the basis of which signals the properties of the paper are determined. The application does not, however, disclose the use of the measurement results in the control and/or regulation of coating mix manufacture.
Patent application WO 99/41590 discloses a method and device for measuring the amount of coating on a moving substrate based on reflection measurement. In accordance with the publication, the amount of calcium carbonate is measured by measuring the absorption peak of calcium carbonate in the wavelength range, which is approximately 3.95 micrometers and/or the amount of kaolin is measured by measuring the absorption peak of kaolin in the wavelength range, which is approximately 2.7 micrometers. According to the publication, the measurement results are used for determining the thickness of the coating and not for controlling or regulating coating manufacture.
In addition, Pentti Niemelä and Janne Suhonen have, in their article “An Industrial Raman Spectrometer for Process Measurement Applications”, presented a measuring device based on Raman spectroscopy for measuring the chemical properties of matter.